I don't know if I could adequately answer the question: "Why would you want to imitate someone elses work?" until the answer came to me in a book.

"Experiencing yourself out of context, divorced from your usual point of view. skews your perspective-it's like hearing your voice on an answering machine. Its almost like meeting a stranger; or discovering a talent you never knew you had. The first time I plucked a melody out on a guitar well enough that it sounded like the original was a bit like that. ..."

This is quoted from the book "Slash" that I just started reading(I hope the quote is OK). That describes the feeling I get each time I get a riff to sound as it should.

edit:
I am only on Chapter 3 so I can't really recommend, yet that anybody else go and buy it. I bought it because the deconstruction successful bands has always interested me. And, you gotta admit Slash seems like a pretty interesting character.